1. Field of the Invention
The present invention relates to a channel scanning method for operating a dual-mode portable terminal device.
2. Description of the Related Art
With various portable telephones used to communicate over radio waves, demand has led to increasing subscribers admitted on the communication system, despite limited frequency bands. In order to meet this need, a conventional analog system called AMPS (Advanced Mobile Phone System) has been in a process of being changed over to an easily operable multiple digital System called CDMA (Code Division Multiple Access).
However, in a period of transition from the analog AMPS to the digital system CDMA, there are still many areas designed to receive both analog and digital services. In such areas it is possible to establish communication with the use of either AMPS or CDMA. Further, a CDMA/AMPS dual-mode portable terminal device has been developed to receive both services.
A conventional CDMA/AMPS dual-mode portable terminal device is designed to operate in the following manner.
FIG. 3 is a schematical block diagram indicating a flow chart of an operation of a conventional CDMA/AMPS dual-mode portable terminal device.
Here, the operation of the conventional CDMA/AMPS dual-mode portable terminal device will be described in accordance with steps S-1 through S-10 in FIG. 3.
Step S-1 (Power ON)
At step S-1, a subscriber (user) in a certain area switches ON his or her CDMA/AMPS dual-mode portable terminal device.
Step S-2 (Initial Setting)
At step S-2, the CDMA/AMPS dual-mode portable terminal device receives a plurality of signals transmitted from a plurality of nearby base stations, and determines which one of the several systems prescribed by International Standard IS-95A, a CDMA spread spectrum standard, is suitable for use with a received signal. Under the International Standard IS-95A, a frequency band used in portable telephone communication is divided into several channels, with each channel being assigned with one of the above systems. After one suitable system is determined and one suitable (i.e., available) channel is caught, the program goes to an Idle state. In this way, a series of operations including the switch-ON of the terminal device, the catching of a suitable or proper channel, the proceeding to the Idle state, is called a channel scan or channel scanning.
Step S-3 (Search in Each PN)
A portable terminal device contains within itself a correlator and a PN code generator. Here, PN code means a series of pseudo-random noise codes which have been set in advance for each channel, or set for each terminal, for diffusibly modulating digital signals. A conventional well-known sliding correlator and a retardation line matching filter may be used. The PN code generator may provide a great number of PN codes (prepared in advance) to the correlator, beginning with a PN code of number 1, number 2, . . . Then, with the use of an output signal from the correlator, the device may determine which PN code is coincident with a PN code contained in a signal transmitted from a nearby base station. If a PN code contained in a signal transmitted from a nearby base station is coincident with a PN code prepared as number K PN code, a PN value of K may be assigned.
The use of a digital CDMA system enables an increase in the number of signals corresponding to the sorts of the PN codes, using only a single channel involving only a single frequency.
Step S-4 (Determination)
If a PN code contained in a received signal is coincident with a PN code fed into the correlator from the PN code generator, the correlator will produce a signal having a level higher than a predetermined threshold level. At this moment, a search result is used as a hit result, and the program goes to step S-5. On the other hand, if the correlator does not produce a signal having a level higher than a predetermined threshold level, the program goes to step S-8.
Step S-5 (Catching a CDMA Channel)
At step S-5, the CDMA/AMPS dual-mode portable terminal device obtains a PN value at a time when the correlator has produced a signal having a level higher than a threshold value, and program goes to step S-6.
Step S-6 (CDMA in Idle State)
Using the above PN value, a desired telephone communication may be conducted through a channel with the use of a digital CDMA system. Namely, with the use of the above PN value, the digital system may catch a channel enabling a desired telephone communication, and the program goes to an Idle state waiting for another call.
Step S-7 (Power OFF)
While in the Idle state waiting for another call is to be released, the user switches OFF the CDMA/AMPS dual-mode portable terminal device.
In the following, an explanation will be given to a condition where the correlator does not produce a signal having a level higher than a predetermined threshold level so that a search fails in catching a suitable channel. At this moment, the program goes to step S-8.
Step S-8 (Catching an AMPS Channel)
At step S-8, the frequency of a signal being received is detected. If the detected frequency equals the frequency using analog AMPS, the channel is thus caught and the program goes to step S-9. Since a signal at this moment corresponds to an analog signal, it is possible to catch a channel of AMPS by tuning a receiver.
Step S-9 (AMPS in Idle State)
By adjusting the above frequency to a receiving frequency, a desired telephone communication through a channel using analog AMPS may be enabled. Namely, catching an analog channel of AMPS permits a desired telephone communication, after which the program goes to an Idle state waiting for another call.
Step S-10 (Power Off)
While in the Idle state waiting for another call is to be released, the user switches OFF the CDMA/AMPS dual-mode portable terminal device.
However, there are some problems with the above process when using the conventional CDMA/AMPS dual-mode portable terminal device.
For example, as shown in FIG. 3, at step S-3, during a search for obtaining a PN value, the PN code generator applies to the correlator a great number of PN codes (prepared in advance), beginning with a PN code of number 1. In this way, a PN search will be performed where a service using the digital system CDMA cannot be received. In order to determine which of the many systems prescribed by the International Standards IS-95A is suitable for treating a received signal, such kind of search is often performed, and approximately 30 seconds are needed to exact a search for a channel of digital system CDMA. As a result, a relatively long time interval occurs between the switching-ON of the terminal device and the arrival at the Idle state for waiting for a call.
Moreover, in areas where a service of digital system CDMA can not be received, the process must transfer to a search for a channel of analog AMPS. This, however, would need a further time interval of 5 seconds. In this way, if the above two kinds of time intervals are added together, a total time interval of approximately 35 seconds would be needed from the switching-ON of the terminal device to the arrival at the Idle state for waiting for a call. During the above time interval, a user of a portable telephone cannot perform either signal transmission or signal receiving, reducing the efficiency of telephone communication.
The International Standard IS-95A is incorporated by reference.
In view of the above discussed problem associated with the above mentioned prior art, it is an object of the present invention to provide an improved channel scanning method for operating a portable terminal device, which method permits a portable terminal device to catch a suitable channel within a short time, reducing wait time, thereby enabling the user to use the portable terminal device with high efficiency.
According to an aspect of the present invention, there is provided a channel scanning method used for a dual-mode portable terminal device capable of handling both an AMPS and a CDMA system, the method comprising:
storing information for identifying a channel caught in a previous Idle state;
storing a PN value used in said previous Idle state, when said channel caught in said previous Idle state corresponds to a CDMA channel;
starting a CDMA channel scan from the CDMA system when the channel caught in a previous Idle state is a CDMA channel;
starting an AMPS channel scan from an AMPS when said channel caught in said previous Idle state is an AMPS channel; and
searching a previously used PN value or a nearby PN value so as to catch a suitable channel.
According to a further aspect of the present invention, the channel scanning method of a portable terminal device, further comprising:
starting said AMPS channel scan when said CDMA channel scan is started from the CDMA system and said previously used PN value or said nearby PN value is searched but the device fails to catch a suitable channel.
According to a still further aspect of the present invention, the channel scanning method of a portable terminal device, further comprising:
starting said CDMA channel scan upon catching said AMPS channel.
According to a still more aspect of the present invention, there is provided a channel scanning method of a portable terminal device responsive to a CDMA system, the method comprising:
storing a PN value used in a previous Idle state; and
starting a channel scan from searching a previously used PN value or a nearby PN value so as to catch a proper channel.